1. Field of the Invention
The present invention relates to liquid crystal displays (LCDs), and particularly to an LCD providing a high transmittance ratio and high response speed of liquid crystal molecules therein.
2. Prior Art
In general, an LCD has two main advantages in comparison with cathode ray tubes (CRTs): LCDs are thin and have low power consumption. It has been said that LCDs might one day completely replace CRT display devices, and LCDs have aroused great interest in many industries in recent times.
However, LCDs generally provide a narrower viewing angle compared with CRT display devices. Various means have been devised in endeavoring to widen the viewing angle of LCDs.
One recently popular of such means is in-plane switching (IPS) mode or fringe field switching (FFS) mode. An LCD using IPS mode is based on a principle different from principles involved in other modes such as Twisted Nematic (TN) mode and Super Twisted Nematic (STN) mode. LCDs using these other modes are hereinafter collectively referred to “usual” LCDs.
In a usual LCD, an electric field is generated perpendicular to a principal surface of a substrate. When the electric field is selectively and locally applied on a liquid crystal layer, liquid crystal molecules in the liquid crystal layer are uniformly oriented perpendicular to the principal surface. Coexistence of these aligned liquid crystal molecules with unaligned liquid crystal molecules provides the visual contrast between black and white on the usual LCD.
On the other hand, the IPS LCD or the FFS LCD provides said contrast by horizontally and locally applying an electric field parallel to a principal surface. That is, the liquid crystal molecules of the liquid crystal layer are locally aligned or oriented along the parallel electric field. Specifically, each liquid crystal molecule, which can be represented by an oval shape, is rotated in a plane due to the parallel field. This results in rotation of an orientation vector of each liquid crystal molecule defined by a long axis of the liquid crystal molecule. The orientation vectors of the liquid crystal molecules are termed directors. The IPS LCD or the FFS LCD can decrease dependency of the viewing angle compared with the usual LCD. Specifically, dependency of the viewing angle is a function of the relationship between the viewing angle and the contrast.
An typical IPS LCD is shown in FIG. 2, the IPS LCD 1 has a first transparent substrate 10, a second transparent substrate 20 opposite to the first transparent substrate 10, a liquid crystal layer 30 sandwiched between the first and the second substrates 10, 20. An electrode array 11 and a first alignment layer 12 are sequentially provided on the first substrate 10 in that order, from bottom to top. The electrode array 11 generally has a common electrode (not shown) and a pixel electrode (not shown), parallel to each other, which are used to produce a electrical field parallel to the two substrates 10, 20. A color filter layer 21 and a second alignment layer 22 are sequentially provided on the second substrate 20 in that order, from top to bottom. The liquid crystal layer 30 has a plurality of liquid crystal molecules.
Generally, for aligning the liquid crystal molecules of the liquid crystal layer 30 in a predetermined direction, two layers of polyimide are respectively coated on the first and second substrates 10, 20; and then the two layers of polyimide are rubbed to respectively form the first and the second alignment layers 12, 22, which have a plurality of grooves thereon having a predetermined orientation direction. The orientation direction of the first alignment layer 12 generally is different from that of the second alignment layer 22. The cooperation of the first and the second alignment layer 12, 22 can realize the alignment of the liquid crystal molecules in the liquid crystal layer 30.
However, because the first and the second alignment layers 12, 22 provide a same anchor force to the adjacent liquid crystal molecules, and the electrical field adjacent to the second substrate 20 is less than the electrical field adjacent to the first substrate 10. Thus, the LCD 1 manufactured by the above mentioned method generally has a question that the liquid crystal molecules adjacent to the second alignment layer 22 don't twist along the electrical field direction or need a long response time to produce twist when voltage is provided on the TFT array 11.
An improved IPS LCD or FFS LCD overcoming the above-mentioned disadvantages is desired.